Designers and engineers provide engine mounts to support the engine and powertrain, and to isolate vibration forces to and from the engine. Engine mounts act to dampen vibrational forces from reciprocating masses in an engine, e.g. a crankshaft, and minimize propagation of vibration from the engine, typically 30-200 Hz, to an engine cradle and chassis. Engine mounts also act to minimize force inputs from the chassis to the engine, e.g. those caused by road surface irregularities, typically less than 30 Hz. Engine mounts serve to isolate the engine from the chassis to improve vehicle driveability, improve customer satisfaction, and improve durability of the engine, the engine mounting system, and the chassis in which the engine is mounted. Mounting devices and systems are often designed to dampen vibrations at specific frequencies, e.g. wherein vibrational inputs may be most severe or most objectionable to an operator. The introduction of displacement-on-demand engine systems introduces new vibrational inputs to the engine mount and vehicle system, due to operating an engine with a bank of cylinders deactivated. There is a change in engine operating characteristics and vibrational frequencies with a change in the number of operating cylinders. Thus, displacement-on-demand engine systems introduce new challenges to the ability of an engine mounting system to control engine vibration.
Conventional passive automotive vehicle powertrain mounts comprise some combination of mass-spring-damper optimized to provide dynamic stiffness and isolation at a key vibrational frequency, while providing less acceptable damping at other frequencies. Conventional mounts exist in many variations and generally operate to provide engine vibration isolation while controlling engine motion with respect to the vehicle frame or body structure.
An exemplary hydraulic mount assembly presently available combines properties of elastomeric materials with hydraulic fluid, and typically includes a reinforced, hollow rubber body that is closed by a resilient diaphragm so as to form a cavity. The cavity is separated into two chambers by a plate. The chambers are in fluid communication through a relatively large central orifice in the plate. A first or primary chamber is formed between the partition plate and the body. A secondary chamber is formed between the plate and the diaphragm. The conventional hydraulic mount assembly may contain a decoupler positioned in the central orifice of the plate that reciprocates in response to vibrations. The decoupler movements accommodate small volume changes in the two chambers. At certain small input vibratory amplitudes and high frequencies, fluid flow between the chambers is substantially avoided and hydraulic damping does not occur. In this manner, the decoupler functions as a passive tuning device.
Engine mount designers have sought to introduce active vibrational tuning devices, wherein an engine mount device includes internal mechanisms to control fluid flow between the chambers in the mount, thus changing dynamic stiffness and other damping characteristics of the mount. Electronic control of the mount is added to be operable to sense vehicle operating conditions, and respond thereto. Such devices often require expensive control mechanisms to effectively operate the device, thus limiting their applicability to high-end vehicle systems.
One such active engine mount device comprises a rubber body with molded in mounting structures, containing fluid and electromagnetic components comprising an electromagnet and piston. The electromagnetic components are driven by external electrical circuits, and when activated, are operable to generate repetitive motion to counteract motion of the engine, thus canceling engine forces from rotational motion. This effectively results in changing the mount stiffness at a specific frequency and therefore changing damping characteristics of the mount at that frequency. Typically, the frequency of interest is the engine cylinder firing frequency. Currently active mount devices are driven with a single-polarity pulse-width modulated signal, which is able to provide a level of vibrational damping.
What is needed is a control scheme for an active mount device which addresses the problems discussed hereinabove, to extend the vibrational damping capability of the active mount device, in order to more effectively dampen engine vibration and provide effective damping in the range of frequencies in which the engine is operating. Extending the range of frequencies over which an active mount device operates is important, especially in conjunction with the use of a displacement-on demand internal combustion engine.